1. Field of the Invention
This invention relates generally to devices that emit electromagnetic radiation and, in particular, to wavelength monitoring and locking for a semiconductor laser.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
Lasers have a wide range of industrial and scientific uses. There are several types of lasers, including gas lasers, solid-state lasers, liquid (dye) lasers, and free electron lasers. Semiconductor lasers are also in use. In semiconductor lasers, electromagnetic waves are amplified in a semiconductor superlattice structure. Semiconductor lasers may be diode lasers (bipolar) or non-diode lasers such as quantum cascade (QC) lasers (unipolar). Semiconductor lasers are used for a variety of applications and can be built with different structures and semiconductor materials, such as gallium arsenide (GaAs).
The use of semiconductor lasers for forming a source of optical energy is attractive for a number of reasons. Semiconductor lasers have a relatively small volume and consume a small amount of power as compared to conventional laser devices. Further, semiconductor lasers can be fabricated as monolithic devices, which do not require a combination of a resonant cavity with external mirrors and other structures to generate a coherent output laser beam.
A semiconductor laser typically comprises an active (optical gain) region sandwiched between two mirrors (reflectors or reflective planes). There is typically a small difference in reflectivity between the two mirrors, one of which (typically, the reflective plane having lower reflectivity) serves as the xe2x80x9cexitxe2x80x9d mirror. The area between the reflective planes is often referred to as the resonator, or the Fabry-Perot resonance cavity in some cases. The active region is located within the resonant cavity. When the active region is pumped with an appropriate pumping energy, it produces photons, some of which resonate and build up to form coherent light in the resonant cavity formed by the two mirrors. A portion of the coherent light built up in the resonating cavity formed by the active region and top and bottom mirrors passes through the exit mirror as the output laser beam.
Various forms of pumping energy may be utilized to cause the active region to begin to emit photons and to achieve gain. For example, semiconductor lasers of various types may be electrically pumped (EP) (by a DC or alternating current), or pumped in other ways, such as by optical pumping (OP) or electron beam pumping. In an EP VCSEL, for example, an electrical potential difference is typically applied across the active region (via top and bottom electrical contacts provided above and below the active region). As a result of the potential applied, a pumping current flows through the active region, i.e. charge carriers (electrons and holes) are injected from opposite directions into the active region where recombination of electron and holes occurs. There are two kinds of recombination events, i.e. radiative and non-radiative, concurrently happening in the active region. When radiative recombination occurs, a photon is emitted with the same energy as the difference in energy between the hole and electron energy states. Some of those photons travel in a direction perpendicular to the reflectors of the laser. As a result of the ensuing reflections, the photons can travel through the active region multiple times.
Stimulated emission occurs when radiative recombination of an electron-hole pair is stimulated by interaction with a photon. In particular, stimulated emission occurs when a photon with an energy equal to the difference between an electron""s energy and a lower energy interacts with the electron. In this case, the photon stimulates the electron to fall into the lower energy state, thereby emitting a second photon. The second photon will have the same energy and frequency as the original photon, and will also be in phase with the original photon. Thus, when the photons produced by spontaneous electron transition interact with other high energy state electrons, stimulated emission can occur so that two photons with identical characteristics are present. (Viewed as waves, the atom emits a wave having twice the amplitude as that of the original photon interacting with the atom.) If a sufficient amount of radiative recombinations are stimulated by photons, the number of photons traveling between the reflectors tends to increase, giving rise to amplification of light and lasing. The result is that coherent light builds up in the resonant cavity formed by the two mirrors, a portion of which passes through the exit mirror as the output laser beam.
Semiconductor lasers may be edge-emitting lasers or surface-emitting lasers (SELs). Edge-emitting semiconductor lasers output their radiation parallel to the wafer surface, while in SELs, the radiation output is perpendicular to the wafer surface. One type of SEL is the vertical-cavity surface-emitting laser (VCSEL). The xe2x80x9cverticalxe2x80x9d direction in a VCSEL is the direction perpendicular to the plane of the substrate on which the constituent layers are deposited or epitaxially grown, with xe2x80x9cupxe2x80x9d being typically defined as the direction of epitaxial growth. In some designs, the output laser beam is emitted out of the top side, in which case the top mirror is the exit mirror. In other designs, the laser beam is emitted from the bottom side, in which case the bottom mirror is the exit mirror.
VCSELs have many attractive features compared to edge-emitting lasers, such as low threshold current, single longitudinal mode, a circular output beam profile, a smaller divergence angle, and scalability to monolithic laser arrays. The shorter cavity resonator of the VCSEL provides for better longitudinal mode selectivity, and hence narrower linewidths. Additionally, because the output is perpendicular to the wafer surface, it is possible to test fabricated VCSELs on the wafer before extensive packaging is done, in contrast to edge-emitting lasers, which must be cut from the wafer to test the laser. Also, because the cavity resonator of the VCSEL is perpendicular to the layers, there is no need for the cleaving operation common to edge-emitting lasers.
The VCSEL structure usually consists of an active (optical gain) region sandwiched between two mirrors, such as distributed Bragg reflector (DBR) mirrors. Both EP and OP VCSEL designs are possible. The two mirrors may be referred to as a top DBR and a bottom DBR. Because the optical gain is low in a vertical cavity design, the reflectors require a high reflectivity in order to achieve a sufficient level of feedback for the device to laser.
DBRs are typically formed of multiple pairs of layers referred to as mirror pairs. DBRs are sometimes referred to as mirror stacks. The pairs of layers are formed of a material system generally consisting of two materials having different indices of refraction and being easily lattice matched to the other portions of the VCSEL, to permit epitaxial fabrication techniques. The layers of the DBR are quarter-wave optical-thickness (QWOT) layers of alternating high and low refractive indices, where each mirror pair contains one high and one low refractive index QWOT layer. The number of mirror pairs per stack may range from 20-40 pairs to achieve a high percentage of reflectivity, depending on the difference between the refractive indices of the layers. A larger number of mirror pairs increases the percentage of reflected light (reflectivity).
The DBR mirrors of a typical VCSEL can be constructed from dielectric (insulating) or semiconductor layers (or a combination of both, including metal mirror sections). The difference between the refractive indices of the layers of the mirror pairs can be higher in dielectric DBRs, generally imparting higher reflectivity to dielectric DBRs than to semiconductor DBRs for the same number of mirror pairs and overall thickness. Conversely, in a dielectric DBR, a smaller number of mirror pairs can achieve the same reflectivity as a larger number in a semiconductor DBR. However, it is sometimes necessary or desirable to use semiconductor DBRs, despite their lower reflectivity/greater thickness, to conduct current, for example (e.g., in an EP VCSEL). Semiconductor DBRs also have higher thermal (heat) conductivity than do dielectric DBRs, making them more desirable for heat-removal purposes, other things being equal. Semiconductor DBRs may also be preferred for manufacturing reasons (e.g., a thicker DBR may be needed for support) or fabrication reasons (e.g., an epitaxial, i.e. semiconductor, DBR may be needed if other epitaxial layers need to be grown on top of the DBR).
When properly designed, these mirror pairs will cause a desired reflectivity at the laser wavelength. Typically in a VCSEL, the mirrors are designed so that the bottom DBR mirror (i.e. the one interposed between the substrate material and the active region) has nearly 100% reflectivity, while the top (exit) DBR mirror has a reflectivity that may be 98%-99.5% (depending on the details of the laser design). The partially reflective top (exit) mirror passes a portion of the coherent light built up in the resonating cavity formed by the active region and top and bottom mirrors. Of course, as noted above, in other designs, the bottom mirror may serve as the exit mirror and the top mirror has the higher reflectivity. VCSELs, DBRs, and related matters are discussed in further detail in Vertical-Cavity Surface-Emitting Lasers: Design, Fabrication, Characterization, and Applications, eds. Carl W. Wilmsen, Henryk Temkin and Larry A. Coldren (Cambridge: Cambridge University Press, 1999).
In standard VCSELs, the active region and top and bottom mirrors are monolithically fabricated on a substrate. A variant on the standard VCSEL, an external cavity VCSEL, or vertical-external-cavity surface-emitting laser (VECSEL), is also in use. In this case, the active region and bottom mirror are monolithically fabricated on a substrate, while the top mirror is mounted externally, some distance (typically very small) above the active region. VECSELs are described in J. Sandusky and S. Brueck, xe2x80x9cA CW External-Cavity Surface-emitting Laser,xe2x80x9d IEEE Photon. Techn. Lett. 8, 313-315 (1996). The term VCSEL may be used to refer to both standard (monolithic) VCSELs and external-cavity VCSELS (VECSELs).
VCSEL characteristics are capable of extensive modeling and manipulation. Sarzala et al., xe2x80x9cCarrier Diffusion Inside Active Regions of Gain-Guided Vertical-Cavity Surface-Emitting Lasers,xe2x80x9d IEEE Proc.xe2x80x94Optoelectonics, vol.144, no.6, p.421-24, December 1997, Langley et al., xe2x80x9cEffect of Optical Feedback on the Noise Properties of Vertical Cavity Surface Emitting Lasers,xe2x80x9d IEEE Proc.xe2x80x94Optoelectonics, vol. 144, no. 1, p. 34-38, February 1997, Ha et al., xe2x80x9cPolarisation Anisotropy in Asymmetric Oxide Aperture VCSELs,xe2x80x9d Electronics Letters, vol. 34, no. 14, July 1998.
Semiconductor lasers such as VCSELs and edge-emitting lasers are used in a variety of applications. In some applications, e.g., telecommunications and spectroscopy among others, the output laser light is modulated to achieve the objective of the system. Modulation consists of modifying a characteristic of the laser output, e.g., the amplitude, frequency, or phase. In the case of telecommunications, the modulations are patterned to correspond to information. The laser may be externally modulated, or directly modulated. When the radiation of the output laser beam is detected after it has traveled to another point, the modulations indicate the information that was encoded at the transmitter/modulator end. A typical telecommunications system uses optical fiber to guide the radiation from the modulation (or emission) point to the detection point. Long wavelength (1.3 xcexcm to 1.55 xcexcm) VCSELs, for example, are of great interest in the optical telecommunications industry because of the minimum fiber dispersion at 1310 nm and the minimum fiber loss at 1.55 xcexcm (1550 nm).
It is important to be able to monitor, and sometimes control, the wavelength of the emitted laser radiation in some applications. In telecommunications applications, for example, the emitted laser radiation of a given semiconductor laser has a precise wavelength, as specified, for example, by the ITU grid. For example, the ITU grid specifies lasing wavelength of 1.55 xcexcm (and other closely spaced wavelengths). These ITU grid wavelengths are used in telecommunications applications such as coarse and dense wavelength-division multiplexing (CWDM and DWDM). In WDM, typically used in optical fiber communications, two or more optical (e.g. laser) signals having different wavelengths are simultaneously transmitted in the same direction over one fiber, and then are separated by wavelength at the distant end.
The use of wavelength-division multiplexed communications systems has led to additional equipment. For example, devices for demultiplexing the wavelengths include the disclosure of U.S. Pat. No. 5,894,535 (1999), Lemoff et al., xe2x80x9cOptical Waveguide Device for Wavelength Demultiplexing and Waveguide Crossing.xe2x80x9d That patent discloses a device including a zigzag patterned dielectric channel waveguide structure that guides a WDM signal through a zigzag path. At particular vertices of the path optical filters selectively transmit and reflect wavelengths of light. The light output of the device separates wavelength of light by output position. As another example, U.S. Pat. No. 5,673,129 (1997), Mizrahi, xe2x80x9cWDM Optical Communication Systems with Wavelength Stabilized Optical Selectorsxe2x80x9d discloses a system that receives a portion of a WDM signal with a Bragg grating having one high reflectivity wavelength band. Based on the signal that is reflected from the grating, a wavelength parameter of the Bragg grating is modified, resulting in a change in the high reflectivity wavelength band. A system basing feedback on the signal transmitted by the grating is also disclosed. U.S. Pat. No. 6,111,681 (2000), Mizrahi et al., xe2x80x9cWDM Optical Communication Systems with Wavelength Stabilized Optical Selectorsxe2x80x9d contains the same disclosure as U.S. Pat. No. 5,673,129.
Systems that adjust the output wavelength of a laser have also been proposed. For example, U.S. Pat. No. 5,943,152, Mizrahi et al., xe2x80x9cLaser Wavelength Control Devicexe2x80x9d discusses a system that couples an in-fiber Bragg grating to the output of a laser. Based on either the transmissivity or reflectance of the grating, a microprocessor continually adjusts the wavelength of the laser output. As another example, U.S. Pat. No. 5,875,273, Mizrahi et al., xe2x80x9cLaser Wavelength Control Under Direct Modulationxe2x80x9d discusses a system using a filter with particular transmission characteristics as a function of wavelength. In particular, the filter includes a transmissivity minimum with transmissivity maximums for both a greater and lesser wavelength, which can also be described as a high reflectivity wavelength band. The filter is coupled to a laser and a control circuit adjusts the laser""s wavelength characteristics based on measurement of both reflected and transmitted light from the filter. As another example, U.S. Pat. No. 6,067,181, Mizrahi, xe2x80x9cLaser Locking and Self Filtering Devicexe2x80x9d discusses a laser system with an optical transfer element and a Bragg grating. The entire output of the laser is coupled to the Bragg grating via the transfer element. The light reflected from the Bragg grating is outputted while the light transmitted through the Bragg grating is detected to generate a signal that is used to control the laser. U.S. Pat. No. 6,125,128 (2000), Mizrahi, xe2x80x9cLaser Output Locking and Self Filtering Devicexe2x80x9d contains substantially the same disclosure as U.S. Pat. No. 6,067,181.
It can be difficult to ensure that a given laser is lasing at the desired wavelength, and to control or tune the laser to emit at different wavelengths. For example, VCSELs can have a wavelength significantly dependent on drive current (or some other tuning parameter), and can be thus said to be xe2x80x9ctunablexe2x80x9d. In general, a tunable laser such as a tunable VCSEL is a laser having an output wavelength corresponding to a selectable tuning parameter. Some approaches used in attempts to tune various types of lasers are described in B. Pezeshki, xe2x80x9cNew Approaches to Laser Tuning,xe2x80x9d Optics and Photonics News, 34-38 (May 2001). These include, in addition to varying the pumping or drive current, temperature variation, combination of multiple lasers having different wavelengths on a single chip, and movement of micromechanical components.
However, while tunability is desired in some applications, it can give rise to undesired variation in lasing wavelength. Additionally, even lasers that initially have a fixed or stable wavelength can have wavelength drift over time, as the device ages. It can be important to be able to determine that desired wavelength, or wavelength xe2x80x9clock,xe2x80x9d has been lost. Information about deviation of the current wavelength from some benchmark or target wavelength can be useful for diagnostic or locking purposes, for example.
There is, therefore, a need for methods and devices to permit monitoring, stabilizing, selecting, and controlling the lasing wavelength of semiconductor lasers.